Ferroelectric element and process for producing the same

ABSTRACT

A ferroelectric element comprising a ferroelectric material containing at least two metals, the ferroelectric element having been produced by a process including a sol-gel process in the presence of a thickener and/or an association preventive from aqueous solutions of respective salts of the metals. The ferroelectric element is advantageous in that the handling of starting compounds and the production of the ferroelectric element are easy, the storage stability of the starting compound solution is good, the cost is low, the ferroelectric element can be formed as a thin layer, particles on the surface of the thin layer are fine and dense, and, hence, the surface smoothness is good. The ferroelectric element is excellent also in piezoelectric properties and therefore can be advantageously used as a piezoelectric element in a piezoelectric ink jet head.

This is a division of application Ser. No. 09/117,377, filed Jul. 29,1998 now U.S. Pat. No. 6,255,762, which is a 371 of PCT/JP96/03745,filed Dec. 20, 1996.

TECHNICAL FIELD

The present invention relates to a ferroelectric element and moreparticularly to a ferroelectric element particularly in the form of athin layer, which is advantageously usable as a piezoelectric element inink jet printers and other devices. The present invention also relatesto a process for producing the ferroelectric element and a ferroelectricprecursor which can be advantageously used in the production of theferroelectric element. Further, the present invention relates to apiezoelectric ink jet head using the above ferroelectric element as apiezoelectric element. The term “piezoelectric” and the term“ferroelectric” used herein are defined as follows. Materials, which,when an external force (a stress from the outside) is applied to acrystal thereof, develop polarization, are called piezoelectrics, and,among the piezoelectrics, those wherein the polarization can be reversedby an external electric field are expressly called ferroelectrics.

BACKGROUND ART

In recent years, in office automation equipment, such as wordprocessors, personal computers, facsimile machines, various measuringinstruments, such as medical measuring instruments, and other devices,ink jet printers have been extensively used for printing informationfrom these devices at high density. As is well known in the art, in theink jet printer, an ink droplet is ejected from a head section of theprinter and deposited directly onto a recording medium, such asrecording paper, to perform monochrome or color printing. The ink jetprinter has many advantages including that printing can be performed oneven a three-dimensional recording medium, running cost is low sinceplain paper can be used as the recording medium, the head can be easilymounted in the printer, the need to provide the step of transfer,fixation and the like can be eliminated, color printing is easilyperformed, and a sharp color printed image can be provided. The headsection of the ink jet printer can be classified into several typesaccording to the drive system for ejecting ink droplets from the headsection. Among them, a typically and advantageously used one is apiezoelectric ink jet head.

The piezoelectric ink jet head generally comprises: a plurality of inkchambers which are disposed at equidistant spaces and function as an inkflow passage and a pressurizing chamber for ejecting an ink; and anozzle plate mounted on the front end of the ink chambers and equippedwith nozzles, for ejecting an ink, corresponding respectively to the inkchambers; and pressing means for pressurizing an ink within the inkchamber in response to the demand for printing. The pressing meanscomprises a piezoelectric element (known also as “piezo element”), andelectrostrictive effect attained by this piezoelectric element isutilized to create a pressure wave within an ink chamber, filled withink, in the head section, permitting the ink to be ejected through thenozzle in the head section.

The structure of the piezoelectric ink jet head will be described inmore detail with reference to FIG. 1. An ink jet head 10, a part ofwhich is shown in the drawing, has an ink chamber member 11 comprising aplurality of ink chambers 12 serving as an ink flow passage and apressurizing chamber for ejecting ink. A nozzle plate (not shown)equipped with nozzles disposed so as to correspond respectively to theink chambers 12 is mounted on the front end of the ink chamber 11. Theink pressurized within the ink chamber 12 can be ejected as a dropletthrough the bore of the nozzle. In the ink chamber member 11 shown inthe drawing, pressing means is mounted on the open face of the inkchamber 12. In the example shown in the drawing, the pressing meanscomprises: a diaphragm 15 for creating a change in volume of the inkchamber 12; a piezoelectric element 17 as a driving element fordistorting the diaphragm 15; and an upper electrode 16 and a lowerelectrode 18 which can apply voltage according to need, thepiezoelectric element 17 being sandwiched between the upper electrode 16and the lower electrode 18.

Ferroelectric elements have been extensively used as the piezoelectricelement for the ink jet head or as an element, for example, forcapacitors, actuators, memories and other elements. The ferroelectricelement consists essentially of a ferroelectric or a ferroelectricmaterial. Typical ferroelectric materials include an oxide ceramicrepresented by the general formula ABO₃ and having a simple perovskitestructure as shown in FIG. 2 and an oxide ceramic having a compositeperovskite structure represented by the general formula (A₁, A₂, . . . )(B₁, B₂, . . . )O₃. The term “perovskite structure” used herein refersto both a simple perovskite structure and a composite perovskitestructure unless otherwise specified. As shown in the drawing, a ceramichaving the above perovskite structure contains metallic ions A and B inthe structure. Examples of more specific ferroelectric materials havingthe above structure include lead zirconate titanate (PZT) represented bythe general formula Pb(Zr, Ti)O₃. In particular, ferroelectrics,containing lead (Pb) as one metal component, including PZT are generallyknown to have large remanence, specific permittivity, and piezoelectricconstant and possesses excellent piezoelectricity and ferroelectricity.In the present specification, the ferroelectric material will bedescribed particularly with reference to PZT.

A sol-gel process has hitherto been well known as a technique for theproduction of PZT, particularly PZT in a thin layer form. Use of thesol-gel process in the production of PZT is advantageous in that ahigh-purity thin layer of PZT can be formed, the composition of theformed thin layer of PZT can reflect the composition of the startingmaterial used, which facilitates the control of the composition and canprovide a thin layer of PZT having high surface smoothness by repetitionof spin coating and firing.

The production of a thin layer of PZT by the sol-gel process and use ofthe thin layer of PZT as a piezoelectric element will be described inmore detail. For example, as described in Japanese Unexamined PatentPublication (Kokai) No. 6-112550, lead acetate is dissolved in aceticacid, and the solution is heated under reflux for 30 min. Zirconiumtetrabutoxide and titanium tetraisopropoxide are then dissolved in thesolution, water and diethylene glycol are added dropwise thereto, andthe mixture is satisfactorily stirred to conduct hydrolysis. To theresultant alcohol solution of a PZT precursor is added polyethyleneglycol monomethyl ether in an amount of 10% by weight based on the PZTprecursor, followed by satisfactory stirring. Thus, a homogeneous sol isprepared. A platinum electrode is formed on a silicon substrate, the solis then spin-coated onto the electrode, and the coating is heated atabout 350° C. Thus, a 2.5 μm-thick, thin, crack-free porous gel layercan be formed.

Subsequently, the same starting material as the above PZT material ishydrolyzed to form a sol. In this case, however, no polyethylene glycolmonomethyl ether is added. The sol is spin-coated onto the above thin,porous gel layer to form a coating which is then dried by heating at400° C. The thin layer thus formed is fired in an oxygen atmosphere for15 hr. The firing temperature is generally 600 to 700° C. Thus, a thinlayer of PZT having a perovskite structure can be formed through theabove series of steps. The above sol-gel reaction may be represented bya general formula as shown in FIG. 3 wherein R represents an alkylgroup.

Further, hydrothermal synthesis has hitherto been well known as a methodfor the formation of a thin layer of PZT from an aqueous solution of amain starting compound. The formation of the thin layer of PZT byhydrothermal synthesis will be described. For example, as described inJapanese Unexamined Patent Publication (Kokai) No. 6-112543, 0.2 mol oflead nitrate, 0.104 mol of zirconium oxychloride, and 0.096 mol oftitanium tetrachloride are dissolved in a 2 N aqueous potassiumhydroxide solution. A silicon substrate with a platinum electrodeprovided thereon is immersed in the solution, and the system is heatedin an autoclave at 160° C. for 30 hr. The substrate is taken out of theautoclave and dried at 200° C. for one hr to form a thin layer of PZT ofcubic particles having an average diameter of 5 μm.

Alternatively, the hydrothermal synthesis may comprise a seed crystalformation process and a crystal growth process. At the outset, in orderto form a seed crystal of PZT, a titanium substrate is immersed in watercontaining lead hydroxide Pb(OH)₂ and zirconium hydroxide Zr(OH)₄, andthe system is heated in an autoclave at a temperature of 140 to 200° C.This heating results in the formation of a thin layer of PZT, capable ofserving as a seed crystal for subsequent layer formation, on the surfaceof the titanium substrate. After the formation of the seed crystal, thetitanium substrate is immersed in water containing Pb(OH)₂, Zr(OH)₄, andtitanium hydroxide Ti(OH)₄, and the system is heated in an autoclave ata temperature of 80 to 150° C. The heating results in the formation of alayer of PZT, in a coarse particle form, having a larger thickness thanthe thin layer of PZT formed above on the thin layer of PZT.

The formation of the thin layer of PZT using the hydrothermal synthesishas advantages including that the layer thickness can be increased at alow temperature of 200° C. or below and an additional step, whichrenders the process complicated, is unnecessary since thepiezoelectricity is developed immediately after the layer formation, andthe adhesion to the substrate is excellent.

The above conventional methods for producing a ferroelectric material,however, involves many problems to be solved. For example, in themethod, described in Japanese Unexamined Patent Publication (Kokai) No.6-112550, wherein the thin layer of PZT is formed by the sol-gel processusing a metal alkoxide as a main starting compound, use of the alcoholas a solvent for the PZT precursor poses a problem that the viscosity ofthe precursor varies depending upon the moisture content of the air,leading to the occurrence of uneven properties of the formed thin layerof PZT. Further, in order to avoid the adverse effect of the moisture inthe air and, therefore, to avoid the formation of insolubilized metalalkoxide, the starting compounds should be mixed together in a specificatmosphere, so that the handling of the starting compounds is not easy.Furthermore, in the sol-gel process, it is difficult to increase thethickness of the thin layer of PZT.

This is true of the hydrothermal synthesis. For example, for the thinlayer of PZT formed by the hydrothermal synthesis described in JapaneseUnexamined Patent Publication (Kokai) No. 6-112543, the average diameterof PZT particles constituting the thin layer is so large that thesurface smoothness of the layer is low and it is difficult to form theupper electrode thereon. Further, the hydrothermal synthesis involvesproblems which include the density of the thin layer being low due tocoarse PZT particles and that potassium (K) is left in the thin layer,adversely affecting the properties.

The present inventors have further made extensive and intensive studiesand, as a result, have found that even use of water instead of thealcohol according to the present invention creates problems in somecases.

As described below in detail, according to the present invention,preferably, three metal salts or metal alkoxides, lead nitrate(Pb(NO₃)₂), zirconium oxynitrate (ZrO(NO₃)₂), and titanium isopropoxide(Ti(O-i-C₃H₇)₄), are used as the starting compound to prepare an aqueousPZT precursor solution. The aqueous PZT precursor solution is coatedonto a predetermined substrate, and the PZT coating is dried and firedto form a thin layer of PZT. In this case, a first possible problem isassociation of lead in the step of drying the PZT precursor coating. Ingeneral, when components, which are different from each other insolubility in water, are mixed together to prepare an aqueous solution,the components contained in the solution associate with each other inthe course of the subsequent step of drying the solution. Thisphenomenon occurs also in the step of drying the PZT precursor coating,and the association of lead is significant. As a result, there is apossibility that a material is significantly precipitated in the form ofcrossed stripes on the surface of the thin layer of PZT. Morespecifically, this is apparent from a microphotograph (magnification:20×) shown in FIG. 4. The creation of the association of lead isconsidered to result in not only the creation of undesired defects onthe surface of the thin layer but also other problems connected with thedefects.

In PZT ceramics, it is known that, regarding the ratio of componentsconstituting the PZT ceramics, a Pb:Zr:Ti:O ratio of 1:0.53:0.47:3offers the highest piezoelectric properties and, when the ratio deviatesfrom this ratio, the piezoelectric properties are rapidly deteriorated.Therefore, in the thin layer of PZT thus formed, even when thePb:Zr:Ti:O ratio is the above desired value in the stage of theprecursor, the association of lead created in the course of drying leadsto an undesired variation of the ratio, resulting in the formation of anuneven layer which provides deteriorated piezoelectric properties. Inaddition, this lowers the density of the layer.

A second possible problem is the creation of defects such as cracks orpinholes. For example, coating of the aqueous PZT precursor solutiononto a substrate by a conventional method, such as dip coating or spincoating, followed by drying, degreasing, and firing to form a thin layerof PZT often creates cracks when the thickness of the thin layer is 1 μmor more. The creation of cracks could not be avoided even though thethin layer of PZT is formed by stacking a plurality of thinner layers ontop of each other or one another. The creation of cracks in the thinlayer of PZT results in lowered density of the layer, making itimpossible to form an element, such as an electrode, on the top of thelayer. Therefore, the formed thin layer of PZT cannot be used, forexample, as a piezoelectric element of an ink jet head. Further, sincethe aqueous PZT precursor solution used in this case has a low viscosityon the order of several centipoises, the coverage per coating is smalland, in addition, pinholes and the like are likely to occur.

In view of the importance of the problem of the creation of cracks orpinholes, the present inventors have made an experiment and, as aresult, have found that coating of an aqueous PZT precursor solutionhaving a composition with the Pb:Zr:Ti:O ratio being 1:0.53:0.47:3 (theabove described preferred ratio) onto a substrate by dip coatingfollowed by drying at 150° C. develops the formation of protrusions of amaterial in a crossed stripe form. After the subsequent firing at 700°C. for crystallization, the protrusions were present, and no noticeabledisappearance of the protrusions was observed. EDX (energy dispersiveX-ray analysis) has revealed that the material in a stripe formconstituting the protrusions has a high lead content. Further, when anaqueous solution of lead, an aqueous solution of zirconium, and anaqueous solution of titanium were prepared as described above, droppedon a substrate and allowed to stand at room temperature, a crystal wasprecipitated only in the aqueous lead solution.

The thickness of the thin layer of PZT formed from the aqueous PZTprecursor solution is 0.05 μm at the largest per coating step, and, inaddition, pinholes were created. Further, when a series of steps ofcoating, drying, degreasing, and firing were repeated ten times, thethickness of the formed thin layer of PZT was 0.5 μm, and, in addition,cracks were created.

DISCLOSURE OF THE INVENTION

Accordingly, a first object of the present invention is to provide aferroelectric element having advantages including that the handling ofstarting compounds and the production of the ferroelectric element areeasy, the storage stability of the starting compound solution is good,the cost is low, the ferroelectric element can be formed as a thinlayer, particles on the surface of the thin layer are fine and dense,and, hence, the surface smoothness is good.

A second object of the present invention is to provide a ferroelectricelement which, in addition to the above properties, when formed as athin layer, permits the layer thickness to be increased and hasexcellent adhesion to the underlying substrate.

A third object of the present invention is to provide a ferroelectricelement which, when formed as a thin layer, does not create any defect,such as a precipitate of a material in a stripe form, on the surface ofthe thin layer, and enables the formation of a dense and even thin layerand, in addition, possesses excellent piezoelectric properties.

A fourth object of the present invention is to provide a ferroelectricelement which, when formed as a thin layer, does not create any defect,such as a precipitate of a material in a stripe form, on the surface ofthe thin layer, enables the formation of a dense and even thin layer,possesses excellent piezoelectric properties, and, in addition, does notcreate defects, such as cracks or pinholes.

A fifth object of the present invention is to provide a process forproducing the above excellent ferroelectric element.

A sixth object of the present invention is to provide a ferroelectricprecursor which can be advantageously used for the production of theabove excellent ferroelectric element.

A seventh object of the present invention is to provide a piezoelectricink jet head comprising the ferroelectric element of the presentinvention as a piezoelectric element.

Other objects of the present invention could be easily understood fromthe following detailed description.

According to one aspect of the present invention, there is provided aferroelectric element comprising a ferroelectric material containing atleast two metals, said ferroelectric element having been produced in thepresence of a thickener and/or an association preventive from aqueoussolutions of respective salts of the metals. Preferably, a sol-gelprocess is used for the preparation of a solution in the formation ofthe element.

In this case, as described above, the ferroelectric material consistingessentially of the ferroelectric element of the present invention ispreferably an oxide ceramic having a simple or a composite perovskitestructure, more preferably lead zirconate titanate (PZT) represented bythe general formula Pb(Zr, Ti)O₃. The PZT ceramic is preferably, but notlimited to, one having a Pb:Zr:Ti:O ratio of 1:0.53:0.47:3 from theviewpoint of good piezoelectric properties and excellent otherproperties. In the present specification, although the practice of thepresent invention will be described particularly with reference to thePZT ceramic, the present invention can be advantageously applied also toother ferroelectric materials.

In the ferroelectric element according to the present invention, thethickener added to the aqueous ferroelectric precursor solution ispreferably a water-soluble polymeric material which can beheat-decomposed when the temperature exceeds a predetermined temperaturein the formation of the element, particularly a water-soluble polymericmaterial which can be heat-decomposed at the time of degreasing or thelike. Suitable thickeners include, but are no limited to, for example,hydroxyalkyl celluloses with the number of carbon atoms in the alkylgroup being preferably 2 to 4, for example, hydroxyethyl cellulose orhydroxypropyl cellulose, polyethylene oxide, and polyvinyl alcohol.These thickener compounds may be used alone or as a mixture of two ormore.

In the ferroelectric element of the present invention, the associationpreventive used, either alone or in combination with the thickener, ispreferably a water-soluble polyhydric alcohol. Suitable polyhydricalcohols include, but are not limited to, for example, diethyleneglycol, polyethylene glycol, and glycerin. These polyhydric alcohols maybe used alone or as a mixture of two or more.

In practicing the present invention, preferably, the thickener and/orthe association preventive are added to the aqueous ferroelectricprecursor solution to permit a powder of a ferroelectric having acrystal structure identical or similar to the ferroelectric to exist.

The ferroelectric element of the present invention is preferably a thinlayer which has been formed from the above ferroelectric precursorsolution through a solution preparation process using a sol-gel process.Although the thin layer generally has a single layer structure, it mayif necessary have a laminate structure of two or more layers.

According to one preferred embodiment of the present invention, asubstrate layer having a crystal structure identical or similar to theferroelectric of the element and formed by hydrothermal synthesis fromaqueous solutions of metals necessary for the formation of the substratelayer may be provided as a layer underlying the ferroelectric element ina thin layer form. In this case, the underlying ferroelectric layer, ascompared with the ferroelectric provided on the ferroelectric layer,comprises particles having a larger diameter and has a lower density.

According to another aspect of the present invention, there is provideda process for producing a ferroelectric element comprising aferroelectric material containing at least two metals, said processcomprising the steps of:

mixing aqueous solutions of respective salts of the metals together toprepare an aqueous ferroelectric precursor solution;

adding a thickener and/or an association preventive to the aqueousferroelectric precursor solution and coating the resultant solution ontoa substrate; and

drying and firing the coating to crystallize the ferroelectric material.

According to still another aspect of the present invention, there isprovided a ferroelectric precursor for use as a starting material in theproduction of a ferroelectric material comprising at least two metals bya process including a sol-gel process, said ferroelectric precursorcomprising aqueous solutions of the respective salts of the metals andcontaining a thickener and/or an association preventive.

According to a further aspect of the present invention, there isprovided an ink jet head comprising a plurality of nozzles for ejectingan ink, ink chambers, for passage and pressurization of the ink,communicating with the nozzles, and pressing means for creating a changein volume of the ink in the ink chamber to eject the ink through thenozzles,

the pressing means comprising a ferroelectric element as a piezoelectricelement, said ferroelectric element comprising a ferroelectric materialcontaining at least two metals and having been produced by a processincluding a sol-gel process in the presence of a thickener and/or anassociation preventive from aqueous solutions of respective salts of themetals.

Basically, the ink jet head of the present invention can have the sameconstruction as the piezoelectric ink jet head commonly used in the artand is not particularly limited so far as the thin layer of theferroelectric is used as the piezoelectric element. Therefore, asuitable ink jet head is, for example, an ink jet head which has beendescribed above with reference to FIG. 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a principal part of aconventional piezoelectric ink jet head;

FIG. 2 is a schematic diagram showing the structure of an oxide ceramichaving a simple perovskite structure;

FIG. 3 is a flow sheet showing a process for producing lead zirconatetitanate (PZT) by the conventional sol-gel process;

FIG. 4 is a microphotograph showing a material in a crossed stripe formformed on the surface of a thin layer of PZT in the formation of thethin layer of PZT by the conventional liquid phase process;

FIG. 5 is a flow sheet showing one preferred embodiment of the processfor producing an aqueous PZT precursor solution according to the presentinvention;

FIG. 6 is a flow sheet showing one preferred embodiment of the processfor producing a thin layer of PZT using as a starting material theaqueous PZT precursor solution prepared according to the process asshown in FIG. 5;

FIG. 7 is a flow sheet showing the process for producing PZT accordingto the present invention;

FIG. 8 is a flow sheet showing one preferred embodiment of the processfor producing a PZT paste, instead of the aqueous PZT precursorsolution, according to the present invention;

FIG. 9 is a cross-sectional view showing the steps of forming a thin,composite PZT layer in sequence according to another preferredembodiment of the present invention;

FIG. 10 is a graph showing an X-ray diffraction pattern of a PZT powderprepared using hydroxypropyl cellulose as a thickener according to thepresent invention;

FIG. 11 is a graph showing an X-ray diffraction pattern of a comparativePZT powder prepared without use of any thickener;

FIG. 12 is a graph showing an X-ray diffraction pattern of a thin layerof PZT prepared using hydroxypropyl cellulose as a thickener accordingto the present invention;

FIG. 13 is a graph showing an X-ray diffraction pattern of a thin layerof PZT prepared using polyethylene oxide as a thickener according to thepresent invention;

FIG. 14 is a graph showing an X-ray diffraction pattern of a thin layerof PZT prepared using polyvinyl alcohol as a thickener according to thepresent invention.

FIG. 15 is a microphotograph showing the surface state of a thin layerof PZT prepared using polyethylene glycol as an association preventiveaccording to the present invention; and

FIG. 16 is a microphotograph showing the surface state of a comparativethin layer of PZT (as a control) prepared using polyethylene glycol asthe association preventive in a reduced amount.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described in moredetail particularly with reference to lead zirconate titanate (PZT). Anembodiment wherein the ferroelectric element of the present invention isused particularly as a piezoelectric element of an ink jet head will bedescribed. However, it should be noted that the ferroelectric elementcan also be advantageously applied to other devices.

The ferroelectric element according to the present invention comprises aferroelectric material containing at least two metals and is produced byusing, as starting materials, aqueous solutions of the respective saltsof metals constituting the ferroelectric. Specifically, a thickenerand/or an association preventive are added to aqueous solutions of metalsalts selected for the ferroelectric, preferably aqueous solutions ofmetal oxides to prepare an aqueous precursor solution, the aqueousprecursor solution is coated, either as such or, if necessary, afterpreparation of a paste from the aqueous precursor solution, onto asubstrate, and the coating is dried and fired.

For example, a thin layer of PZT, which is a representative example ofthe ferroelectric element of the present invention, may be formed bypreparing an aqueous solution of a salt of lead, an aqueous solution ofa salt of zirconium, and an aqueous solution of a salt of titanium,mixing these aqueous solutions together to prepare an aqueous PZTprecursor solution, coating the aqueous precursor solution onto apredetermined substrate, and then subjecting the coating to treatmentsuch as drying and firing.

The above series of steps for forming the thin layer of PZT aredescribed in more detail in FIGS. 5 and 6 (flow sheets) and will bedescribed below in sequence. One preferred embodiment of each step willbe described, but it will be understood that variations andmodifications can be effected within the spirit and scope of theinvention.

For the formation of a thin layer of PZT described herein, three metalsalts, lead nitrate (Pb(NO₃)₂), zirconium oxynitrate (ZrO(NO₃)₂), andtitanium isopropoxide (Ti(O-i-C₃H₇)₄), are used as the startingcompound.

Preparation of Aqueous Titanium Solution

A predetermined amount of titanium isopropoxide (Ti(O-i-C₃H₇)₄) isdissolved in a 3 N aqueous nitric acid solution, and the solution isstirred in a nitrogen atmosphere to perform mixing. The reason why themixing is performed in a nitrogen atmosphere is that, when the mixing isperformed in air, titanium isopropoxide is insolubilized through areaction with moisture in air to produce TiO₂. The mixing in a nitrogenatmosphere gives a homogeneous aqueous titanium solution through thefollowing reaction.

Ti(O-i-C₃H₇)₄+2H₂O→Ti(OH)₄+4C₃H₇OH

Preparation of Aqueous Zirconium Solution

A predetermined amount of zirconium oxynitrate is dissolved in purewater, and the solution is stirred to perform mixing. The mixing gives ahomogeneous aqueous zirconium solution through the following reaction.

ZrO(NO₃)₂+2H₂O→Zr(OH)₄+2HNO₃

Mixing

The aqueous titanium solution and the aqueous zirconium solutionprepared above are mixed together while stirring to prepare ahomogeneous solution.

Preparation of Aqueous Lead Solution

A predetermined amount of lead nitrate is dissolved in pure water, andthe solution is stirred to perform mixing. The mixing gives ahomogeneous aqueous lead solution through the following reaction.

Pb(NO₃)₂+2H₂O→Pb(OH)₂+2HNO₃

In this case, for example, another salt, such as lead acetate, may beused instead of lead nitrate.

Preparation of Aqueous PZT Precursor Solution

The aqueous lead solution thus prepared is mixed with the mixed aqueoussolution, containing titanium and zirconium, prepared above to prepare ahomogeneous aqueous PZT precursor solution. The concentration of theaqueous PZT precursor solution is not particularly limited and may bewidely varied by taking desired results, film forming conditions andother various conditions, into consideration. The present inventors havefound that the concentration of PZT in the aqueous PZT precursorsolution is preferably about 5 to 20% by weight, more preferably about10% by weight. The aqueous PZT precursor solution, as compared with theconventional alcohol solution of the PZT precursor, is easy to handleand is inexpensive. Thus, the series of steps shown in FIG. 5 arecompleted. Subsequently, the step of forming a thin layer of PZT (seeFIG. 6) is carried out.

Addition of Thickener and/or Association Preventive

A thickener and an association preventive are added, either alone or incombination, to the aqueous PZT precursor solution prepared aboveaccording to the present invention. These additives may be added in anyorder. The thickener and the association preventive will be described inmore detail below.

After the addition of the necessary additive(s) has been completed, themixture is thoroughly stirred. Thus, a coating solution for a thin layerof PZT is prepared. If desired, the coating solution may be treated toprepare a paste.

Coating (Formation of Coating)

The coating solution or the paste prepared above is coated on apredetermined substrate to form a desired pattern. The coating may beperformed by a conventional method, and examples of coating methodsusable herein include spin coating, dip coating, and screen printing. Anoptimal coating method may be selected according to the type ofcontemplated thin layer of PZT. Likewise, the coverage, that is, thethickness of the coating formed, may be suitably selected according tovarious factors. For example, a submicron layer thickness after dryingsuffices for use of the resultant thin layer of PZT as a memory, acapacitor and the like. On the other hand, a layer thickness on theorder of several tens of μm suffices for use of the thin layer of PZT asan actuator.

Drying

Subsequently, the coating thus formed is dried to cure the coating and,at the same time, to remove excess water or the like by evaporation. Thedrying temperature and time may be widely varied. In general, however,the drying is performed at about 100 to 200° C. for about 5 to 30 min.For example, drying at 150° C. for 10 min may be adopted. Defects, suchas cracks, are not created in the coating in the course of the step ofdrying. Further, the step of degreasing and other conventional treatmentsteps (not shown) may be interposed between the step of drying and thestep of firing.

Firing

Finally, the dried coating is fired. As with the step of dryingdescribed above, the step of firing may be carried out by any methodcommonly used in the art. The firing temperature is preferably about 500to 900° C., more preferably about 700 to 800° C. In general, firing at atemperature around 500° C. can offer the desired results. The firingtime may be widely varied according to the relationship thereof with thefiring temperature or the like. In general, however, it is in the rangeof about 1 to 60 min. A thin layer of PZT, which has high density and isvery high compact, is formed as a result of the firing. As describedabove, when the firing temperature is low, a problem of scattering oflead (Pb) during the firing can be avoided. Therefore, a high-qualitythin layer of PZT can be obtained while maintaining the same composition(Pb:Zr:Ti:O ratio) as that of the PZT precursor provided above.

The state in the course of the formation of the thin layer of PZT isshown in FIG. 7. From comparison of FIG. 7 with FIG. 3 described above,it will be understood that the structure of the gel according to thepresent invention is distinguished from that of the conventional gel.

In the formation of the above thin layer of PZT, the Pb:Zr:Ti:O ratio inthe PZT precursor may be widely varied according to the desired results.In order to provide the best piezoelectric properties, as describedabove, it is recommended that Pb:Zr:Ti:O be 1:0.53:0.47:3. In thepresent invention, when the above composition ratio is adopted in thePZT precursor, the ratio may be reproduced as it is in the thin layer ofPZT, enabling a thin layer of PZT, which is homogeneous, has highdensity and possesses excellent piezoelectric properties, to be easilyprovided.

Regarding the creation of defects, such as cracks or pinholes, describedabove in connection with the prior art, according to the presentinvention, addition of a PZT powder in addition to the thickener and/orthe association preventive in the stage of preparing the aqueous PZTprecursor solution can effectively prevent the creation of the defects.In this case, preferably, the PZT precursor is in the form of a pasterather than an aqueous solution.

FIG. 8 shows a flow sheet of this preferred embodiment. The proceduredescribed above with reference to FIG. 5 may be repeated up to the stepat which the aqueous PZT precursor solution is prepared. Next, asdescribed above with reference to FIG. 6, the thickener and/or theassociation preventive are added. The PZT powder is added simultaneouslywith, before or after the addition of these additives. The order ofadding the above additives may be properly varied according to thedesired results and other factors. PZT powders usable herein will bedescribed in more detail below.

After the addition of the necessary additives has been completed, themixture is placed in a milling device, for example, a planetary ballmill, followed by milling for several minutes. If necessary, theaddition of the PZT powder may be carried out in the step of milling.Thus, a homogeneous paste can be obtained which is then coatable byscreen printing or the like. Preferably, after vacuum deaeration forseveral min by means of a rotary pump, the paste is coated. The steps ofcoating, drying, and firing may be carried out in the same manner asdescribed above with reference to FIG. 6.

In the practice of the present invention, the aqueous PZT precursorsolution may contain a thickener. Preferably, the thickener may be addedafter the preparation of the aqueous precursor solution. Functions ofthe thickener include, for example, an improvement in crystallinity ofthe thin layer of PZT and, when the PZT powder is also used, homogeneousdispersion of the PZT powder. The thickener is often called a binderfrom the viewpoint of the function. A suitable thickener is awater-soluble polymeric material which can be heat-decomposed when thetemperature exceeds a predetermined temperature in the formation of theelement (that is, at the time of firing). Examples of thickeners whichcan be advantageously used in the present invention include, but are notlimited to, hydroxyalkyl celluloses with the number of carbon atoms inthe alkyl group being preferably 2 to 4 (for example, hydroxypropylcellulose), polyethylene oxide, and polyvinyl alcohol. These thickenercompounds may be used alone or as a mixture of two or more. The amountof the thickener used may be widely varied according to factors such asthe desired effects. In general, however, the amount is about 0.1 to 50%by weight based on the total amount of the aqueous PZT precursorsolution. Preferably, when the thickener used is hydroxypropylcellulose, the amount thereof is generally 0.5 to 10% by weight based onthe total amount of the aqueous PZT precursor solution. When the purposeof adding the thickener is to homogeneously disperse the PZT powder, theamount of the thickener added may be relatively large.

In the practice of the present invention, the association preventive isused, either in combination with the thickener or independently of thethickener, in the aqueous PZT precursor solution. The associationpreventive, when contained in the aqueous precursor solution, can becoordinated to a lead (Pb) element contained in the aqueous solution toeffectively prevent the association of lead elements with each otherwhich has been the problem of the prior art. A suitable associationpreventive is a water-soluble polyhydric alcohol, and examples ofpolyhydric alcohols, which can be advantageously used in the presentinvention, include, but are not limited to, diethylene glycol,polyethylene glycol, and glycerin. These association preventives may beused either alone or in combination. The amount of the associationpreventive used may be widely varied according to factors such as thedesired effects. In general, however, the amount is about 5 to 20% byweight based on the total amount of the aqueous PZT precursor solution.

In the present invention, it is possible to attain not only the effectsinherent in the thickener and the association preventive by the additionof these additives but also the effect, described above, by thedissolution of the PZT precursor in water instead of an alcohol. Thatis, the problem of the variation in viscosity of the precursor due tothe moisture in the air, which is experienced in the use of an alcoholas the solvent, can be avoided. Therefore, the precursor can be easilyused in a very stable state. Practice of the sol-gel process using thisprecursor results in the formation of a thin layer of PZT which is denseand possesses excellent piezoelectric properties.

In the practice of the present invention, when there is a fear of cracksor pinholes being created due to the use of the aqueous PZT precursorsolution, or even though the above fear does not exist, addition ofanother PZT powder having a crystal structure identical or similar tothe PZT, in addition to the addition of the thickener and/or theassociation preventive, to the aqueous PZT precursor solution ispreferred. The PZT powder added in this case can simultaneously exhibitvarious noteworthy functions. For example, the PZT powder, when the PZTcoating after drying is fired, can reduce the shrinkage of the coatingby the volume of the powder added, leading to the prevention ofcracking. Further, addition of the PZT powder can increase the coverageof PZT per coating step and hence can increase the thickness of the thinlayer of PZT. The amount of the PZT powder added may widely varyaccording to factors such as the desired effects. In general, however,the amount is about 5 to 20% by weight based on the total amount of theaqueous PZT precursor solution. The above effects have been describedparticularly with reference to the addition of a PZT powder. Also in theproduction of ferroelectric elements other than PZT, addition of acorresponding ferroelectric powder can provide effects comparablefavorably with the above effects.

Further, in the practice of the present invention, as described above,addition of the thickener in combination with the PZT powder ispreferred. Addition of the thickener enables the viscosity of the PZTprecursor paste to be controlled and hence can control the thickness ofthe resultant thin layer of PZT. Addition of the additives followed byagitating operation, such as milling, permits the mixed PZT powder to behomogeneously dispersed. Further, settling of the PZT powder can beeffectively prevented. Furthermore, the addition of the thickener canimprove the wettability of the substrate by the PZT paste and theadhesion of the thin layer of PZT after firing to the substrate. Whenthe thickener is used in combination with the PZT powder, the amount ofthe thickener used may vary widely depending upon the desired viscosity,the desired effects and other factors in the paste. In general, however,the amount is about 1 to 30% by weight based on the total amount of theaqueous PZT precursor solution.

The addition of additional additives brings the PZT precursor as thestarting material in the form of a solution to a paste. The resultantPZT paste may be coated by the above-described methods, such as spincoating and dip coating. When the viscosity is taken into consideration,however, coating may be advantageously performed by screen printing andother methods commonly used in the field of coating formation. Use ofthe screen printing enables the PZT coating to be formed in a desiredpattern and, at the same time, can facilitate increasing the coatingthickness.

Thus, addition of the PZT powder, in addition to the thickener and/orthe association preventive, to the aqueous PZT precursor can provide aPZT paste which has excellent storage stability and high PZTconcentration. Use of this paste results in the formation of a thinlayer of PZT which is free from defects, such as pinholes and cracks,and possesses excellent piezoelectric properties.

According to another preferred embodiment of the present invention, aferroelectric element, such as a thin layer of PZT, is produced bycombining hydrothermal synthesis with the sol-gel process. In this case,the hydrothermal synthesis is basically used for the formation of afirst layer (that is, a ferroelectric substrate layer) of theferroelectric element by the conventional method. On the other hand, thesol-gel process is basically used for the formation of a second layer(an upper ferroelectric layer) of the ferroelectric element using as astarting material an aqueous ferroelectric precursor solution containingat least a thickener and/or an association preventive and optionally aferroelectric powder.

That is, the ferroelectric element according to this preferredembodiment is characterized in that the upper ferroelectric layerconstituting the main part of the ferroelectric element has, on theunderside thereof, the ferroelectric substrate layer which has a crystalstructure identical or similar to the ferroelectric materialconstituting the upper ferroelectric layer and has been formed, fromaqueous solutions of metals necessary for the formation of theferroelectric substrate layer, by hydrothermal synthesis. In the abovecomposite ferroelectric element, preferably, the ferroelectric materialfor the ferroelectric substrate layer, as compared with theferroelectric material for the upper ferroelectric layer, is constitutedby particles having a larger diameter and has lower density.

Preferably, the above composite ferroelectric element, for example, acomposite thin PZT layer, may be produced, for example, by a processcomprising steps shown in sequence in FIG. 9.

At the outset, as shown in the step A, a substrate 1 is provided. Forthe substrate, the type, shape and the like may widely vary dependingupon the contemplated application of the composite thin PZT layer andother factors. Suitable substrates include, for example, conventionalsubstrates, such as ceramic substrates, for example, silicon substratesand titanium substrates, and glass substrates. The substrate may havethereon a layer, such as an insulating layer, wiring, or an electrode.In the present embodiment, a titanium substrate is used.

Next, in the step B, a seed crystal layer 2 of PZT is formed on thetitanium substrate 1. The seed crystal layer 2 may be formed, forexample, by immersing the titanium substrate 1 in water containing leadhydroxide (Pb(OH)₂) and zirconium hydroxide (Zr(OH)₄) and heating thesystem in an autoclave at a temperature of 140 to 200° C.

After the formation of the seed crystal layer 2 has been completed, inthe step C, a first PZT layer (substrate layer) 3 is further formed onthe seed crystal layer 2. This PZT layer 3 may be formed, for example,by immersing the titanium substrate 1, with the seed crystal layer 2formed thereon, in water containing Pb(OH)₂, Zr(OH)₄, and titaniumhydroxide (Ti(OH)₄) and heating the system in an autoclave at atemperature of 80 to 150° C. As a result, on the seed crystal layer 2formed above is formed a PZT layer 3, as shown in the drawing, which, ascompared with the seed crystal layer 2, is constituted by coarserparticles and has larger layer thickness. The PZT layer 3 isadvantageous in that an increase in the layer thickness is possible at alow temperature and, in addition, the PZT layer 3 can developpiezoelectricity immediately after the layer formation and has goodadhesion to the titanium substrate 1.

Finally, in the step D, a second PZT layer 4 is formed using the aqueousPZT precursor solution or the paste according to the present invention.Disposition of the PZT layer 4 on the first PZT layer 3 can smoothen theroughened surface of the underlying PZT layer 3. Since the surface ofthe composite thin PZT layer is smooth, an electrode and the like may beeasily formed thereon with high reliability. In the case of the abovetwo-layer structure, the creation of defects, such as cracks orpinholes, can be prevented in the composite thin PZT layer.

The ferroelectric element according to the present invention can beadvantageously used for constituting pressing means in a piezoelectricink jet head, that is, an ink jet head comprising: a plurality ofnozzles for ejecting an ink; an ink chamber, communicating with thenozzles, for flow and pressurization of the ink; and pressing means forcreating a change in volume of the ink in the ink chamber to eject theink through the nozzle.

As described above, the ink jet head per se can have the sameconstruction as the piezoelectric ink jet head commonly used in the art.The ink jet head of the present invention will be described again withreference to FIG. 1.

An ink chamber 12 in an ink jet head 10 is provided in a predeterminedpattern in an ink chamber member 11. The ink chamber member 11 may bemade of various materials according to factors such as the method forforming the ink chamber 12. One embodiment of the ink chamber member 11will be described. For example, glass, a plastic material, for example,a polyester resin (such as PET), an acrylic resin (such as PMMA),quartz, or other substrate is provided, and a resin material may be thenpatterned onto the substrate by photolithography or another method toform a groove corresponding to the ink chamber.

In the ink chamber member 11 shown in the drawing, a diaphragm 15 forcreating a change in volume of the ink chamber 12 (one constituentelement of pressing means referred to in the present invention) isprovided on the open face of the ink chamber 12. The diaphragm 15 may beintegrally molded with the ink chamber member 11 at the time offormation of the ink chamber member 11. Otherwise, it may be bonded tothe ink chamber member 11 with the aid of an adhesive. Suitablematerials for the diaphragm 15 include, for example, ceramics havinghigh rigidity, for example, ZrO₂. The remaining elements of the pressingmeans are attached so as to abut against the diaphragm 15.

In the case of the embodiment shown in the drawing, the remainingelements of the pressing means are a piezoelectric element 17 as a drivefor distorting the diaphragm 15 and an upper electrode 16 and a lowerelectrode 18 which sandwich the piezoelectric element 17 therebetweenand, when necessary, can apply a voltage. The piezoelectric element 17may be formed by the above process according to the present invention.The electrodes 16 and 18 each may be formed in a desired pattern by aconventional method, for example, sputtering, screen printing, or vapordeposition.

The ink jet head shown in the drawing may be operated as follows. An inkis fed into the ink chamber 12 through an ink feed port (not shown) ofthe head 10 to fill the ink chamber 12 with the ink. In this state,application of a voltage across the electrodes 16 and 18 creates adisplacement of the piezoelectric element 17 sandwiched between theseelectrodes due to the piezoelectric properties, permitting the diaphragm15 disposed so as to abut against the piezoelectric element 17 to bepushed out towards the ink chamber 12. This reduces the volume of theink filled into the ink chamber 12, leading to ejection of the ink in adot form in an amount corresponding to the reduction of the volumethrough a nozzle (not shown) communicating with the ink chamber 12.

EXAMPLES

The present invention will be described in more detail with reference tothe following examples. It should be understood that the presentinvention is not limited to these examples only.

Example 1

0.014 mol of titanium tetraisopropoxide was dissolved in 30 g of a 3 Naqueous nitric acid solution to prepare an aqueous titanium solution.Separately, 0.016 mol of zirconium oxynitrate was dissolved in 20 g ofpure water to prepare an aqueous zirconium solution. Further, 0.03 molof lead nitrate was dissolved in 20 g of pure water to prepare anaqueous lead solution.

The three aqueous solutions thus prepared were mixed together to preparean aqueous PZT precursor solution. In this connection, it should benoted that the ratio of Zr to Ti in PZT is closely related to theferroelectricity. Therefore, in the preparation of the aqueous solutionsfollowed by mixing of these aqueous solutions, this has been taken intoconsideration, and the preparation and mixing were performed so that thefinally formed thin layer of PZT had a desired composition ratio(Pb:Zr:TiO=1:0.53:0.47:3),

At the outset, the aqueous titanium solution and the aqueous zirconiumsolution were thoroughly mixed together at room temperature whilestirring. After the completion of the stirring, the mixture was mixedwith the aqueous lead solution. The resultant mixture was thoroughlystirred at room temperature and then heated under reflux at atemperature of 100° C. or above to conduct hydrolysis. Metal ionscontained in each aqueous solution, when the aqueous solutions weremixed together at room temperature, merely formed corresponding metalhydroxides, and the degree of bonding among the metal ions was small.Subsequent reflux and hydrolysis, however, permit the metal ions, thatis, titanium, zirconium, and lead ions, to be bonded to one anotherthrough oxygen. In this case, a Pb—O—Ti—O'Zr bond is formed to bring thesystem to a sol. In the present invention, the aqueous solution in thissol state is expressly called an “aqueous PZT precursor solution.”

After the temperature of the aqueous PZT precursor solution was returnedto room temperature, water and a small amount (about 0.5% by weight) ofhydroxypropyl cellulose were added thereto, followed by thorough mixingwith stirring at room temperature for about one hour. As a result, atransparent, homogeneous solution was obtained which was neither cloudynor contained any precipitate.

In this connection, it was confirmed that hydroxypropyl cellulose addedto the aqueous PZT precursor solution has various effects. Specifically,hydroxypropyl cellulose increases the viscosity of the aqueous PZTprecursor solution having relatively low viscosity and, when the aqueousPZT precursor solution is coated on the substrate in the step ofcoating, enables the formation of a coating having large thickness. Notonly has hydroxypropyl cellulose the effect of increasing the viscosity,but also the effect of strengthening the Pb—O—Ti—O—Zr bond and theeffect of reducing the distance among the metal ions. Although thereason why these effects can be attained has not been fully elucidatedyet, according to studies conducted by the present inventors, it isbelieved that the metal ions and oxygen are intertwined withhydroxypropyl cellulose as the backbone, leading to the above effects.As a result, the firing temperature in the layer formation can belowered, resulting in improved crystallinity of the thin layer.

The results of an experiment conducted by the present inventors toconfirming the above excellent effects of hydroxypropyl cellulose willbe described.

The aqueous PZT precursor solution prepared as described above wasplaced in a crucible and fired at a predetermined temperature for agiven period of time. As a result, a light-yellow PZT powder (sample 1)was obtained. For comparison, a light-yellow PZT powder (sample 2) wasprepared in the same manner as described above, except thathydroxypropyl cellulose (thickener) was not used. For the samples, theX-ray diffraction pattern was measured. The results were as shown inFIG. 10 (sample 1) and FIG. 11 (sample 2).

In the X-ray diffraction patterns shown in FIGS. 10 and 11, a peakaround an X-ray diffraction angle θ=29° is one, derived from apyrochlore layer which appears when the crystallization of PZT isunsatisfactory. The crystallinity of PZT can be quantitativelydetermined by comparing the intensity of this peak with the peakintensity of the (101) face having the highest diffraction intensityamong the PZT diffraction peaks. For sample 1 (FIG. 10) which is anexample of the present invention, the proportion of the pyrochlore layerwas 1.3%, whereas for sample 2 (FIG. 11) which is a comparative example,the proportion of the pyrochlore layer was 11.6%. This indicates thataddition of hydroxypropyl cellulose as the thickener can provide a morecomplete PZT crystal.

Example 2

A thin layer of PZT was formed from the aqueous PZT precursor solutionprepared in Example 1, and the thin layer of PZT was incorporated as apiezoelectric element into a piezoelectric ink jet head having astructure shown in FIG. 1.

A diaphragm of a thin ceramic sheet was prepared, and a platinumelectrode was formed in a predetermined area by sputtering. Further, theaqueous PZT precursor solution prepared in Example 1 was spin-coated at2000 rpm onto the platinum electrode. The PZT coating on the platinumelectrode was dried at 150° C. for 10 min and further pre-fired at 550°C. for one hr. As a result of the pre-firing, hydroxypropyl cellulosecontained in the PZT coating was decomposed and scattered.

The PZT coating on the platinum electrode was then fired at 700° C. forone min. This allowed the crystallization of PZT to proceed and a thinlayer of PZT having a perovskite structure was obtained. This thin layerof PZT had good adhesion to the platinum electrode and the diaphragm andfurther exhibited satisfactory adhesion in the sputtering of anadditional platinum electrode on the thin layer of PZT. Further, since arelatively low temperature can be applied to the firing of the thin filmof PZT, the range of selection of the underlying diaphragm can bebroadened.

FIG. 12 shows an X-ray diffraction pattern of the thin layer of PZT thusformed. In the X-ray diffraction pattern, no diffraction peak other thanpeaks derived from PZT is observed, indicating that the layer formed onthe platinum electrode is a thin layer of PZT.

Example 3

The procedure of Example 2 was repeated, except that, in the preparationof the aqueous PZT precursor solution in the same manner as in Example1, polyethylene oxide (sample 3) and polyvinyl alcohol (sample 4) wereused as a thickener instead of hydroxypropyl cellulose. As a result,satisfactory thin layers of PZT comparable with the thin layer of PZTformed in Example 2 could be formed.

The X-ray diffraction patterns of the thin layers of PZT thus formedwere as shown in FIG. 13 (sample 3) and FIG. 14 (sample 4). Thethickness of the thin layers of PZT was so small that, in these X-raydiffraction patterns, peaks derived from the platinum electrode could beobserved around diffraction angles, 2θ, of 40°, 47°, and 67°. Peaksother than the peaks derived from platinum were only those derived fromPZT, indicating that the formed layer was a thin layer of PZT.

Example 4

0.014 mol of titanium tetrachloride was dissolved in aqueous ammonia,and the resultant titanium hydroxide was collected by filtration andwashed. The titanium hydroxide was dissolved in a 3 N aqueous nitricacid solution to prepare an aqueous titanium solution. Separately, 0.016mol of zirconium oxynitrate was dissolved in 20 g of pure water toprepare an aqueous zirconium solution. Further, 0.03 mol of lead nitratewas dissolved in 20 g of pure water to prepare an aqueous lead solution.

The three aqueous solutions thus prepared were mixed together to preparean aqueous PZT precursor solution. At the outset, the aqueous titaniumsolution and the aqueous a zirconium solution were thoroughly mixedtogether at room temperature while stirring. After the completion of thestirring, the mixture was mixed with the aqueous lead solution. Theresultant mixture was thoroughly stirred at room temperature and thenheated under reflux at a temperature of 100° C. or above to conducthydrolysis. After the temperature of the resultant aqueous PZT precursorsolution was returned to room temperature, water and a small amount(about 0.5% by weight) of hydroxypropyl cellulose were added thereto,followed by thorough mixing with stirring at room temperature for aboutone hr. As a result, a transparent, homogeneous solution was obtainedwhich was neither cloudy nor contained any precipitate.

Example 5

A thin layer of PZT was formed from the aqueous PZT precursor solutionprepared in Example 4, and the thin layer of PZT was incorporated as apiezoelectric element into a piezoelectric ink jet head having astructure shown in FIG. 1.

A diaphragm of a thin ceramic sheet was prepared, and a platinumelectrode was formed in a predetermined area by sputtering. Further, theaqueous PZT precursor solution prepared in Example 4 was spin-coated at2000 rpm onto the platinum electrode. The PZT coating on the platinumelectrode was dried at 150° C. for 10 min and further pre-fired at 500°C. for one hr. As a result of the pre-firing, hydroxypropyl cellulosecontained in the PZT coating was decomposed and scattered.

The PZT coating on the platinum electrode was then fired at 650° C. forone hr. This allowed the crystallization of PZT to proceed and a thinlayer of PZT having a perovskite structure was obtained. This thin layerof PZT had good adhesion to the platinum electrode and the diaphragm andfurther exhibited satisfactory adhesion in the sputtering of anadditional platinum electrode on the thin layer of PZT.

Example 6

In this example, lead nitrate (Pb(NO₃)₂), zirconium oxynitrate(ZrO(NO₃)₂), and titanium isopropoxide (Ti(O-i-C₃H₇)₄) were used asstarting compounds.

At the outset, 4.03 g of titanium isopropoxide was dissolved in andmixed with 23.3 g of a 2.8 N aqueous nitric acid solution while stirringin a nitrogen atmosphere to give an aqueous titanium solution.Separately, 4.27 g of zirconium oxynitrate was dissolved in and mixedwith 18 g of pure water while stirring to give an aqueous zirconiumsolution. After the preparation of the aqueous titanium solution and theaqueous zirconium solution, these aqueous solutions were mixed togetherwhile stirring to prepare a homogeneous solution. Separately, 10 g oflead nitrate was dissolved in and mixed with 32 g of pure water whilestirring. The resultant aqueous lead solution was then mixed with themixed aqueous solution containing titanium and zirconium while stirring.Finally, 10% by weight, based on the resultant mixture, of polyethyleneglycol (weight average molecular weight=200) was added to the mixture.Thus, a homogeneous aqueous PZT precursor solution was prepared.

In the aqueous PZT precursor solution prepared above, the PZTconcentration was 9% by weight, and the composition (molar ratio ofmetal elements contained) was Pb:Zr:Ti=1:0.53:0.47. Among a number ofPZTs, the PZT having this molar ratio offered the maximum valuesrespectively for the permittivity, the piezoelectric constant and thelike.

The aqueous PZT precursor solution prepared above was then spin-coatedon a Pt/Ti/Si substrate, and the coating was dried at 150° C. for 10min, pre-fired at 500° C. for 60 min, and fired at 800° C. for 60 min.As a result, a dense thin layer of PZT having a thickness of 100 nm wasformed. The thin layer of PZT was examined, and, as a result,precipitation of a material in a stripe form on the surface of the thinlayer was not observed.

Further, in order to evaluate a change in the aqueous PZT precursorsolution with the elapse of time, the aqueous PZT precursor solution wasallowed to stand in air at room temperature for one month. As a result,no change occurred. Since polyethylene glycol used as the associationpreventive in this example has already been found, by TG-DTA(thermogravimetry-differential thermal analysis), to be a material whichcould be degreased at 300° C., it is considered that degreasing could befully achieved in the step of pre-firing.

Example 7

The procedure of Example 6 was repeated, except that, in the preparationof the aqueous lead solution, lead acetate, instead of lead nitrate, wasused in the same amount. As a result, a satisfactory thin layer of PZTcomparable with the thin layer of PZT formed in Example 6 was formed.

Example 8

In order to evaluate the influence of an association preventive on lead(Pb) in an aqueous PZT precursor solution, the procedure of Example 6was repeated, except that the amount of polyethylene glycol used as theassociation preventive was reduced from 10% by weight to 5% by weight.As a result, unlike Example 6 (amount of association preventive used:10% by weight) wherein no association of lead was observed, in thisexample (amount of association preventive used: 5% by weight), theassociation of lead was significant.

The above results indicate that, since polyethylene glycol used here hasan average molecular weight of 200 and a monomolecular weight of 63, alarge amount of tetramer exists. In this case, regarding the length ofthe molecule, Pb²⁺ is 1.2 Å, while polyethylene glycol in a tetramerform is several tens of Å. That is, the is polyethylene glycol moleculeis considerably longer than Pb²⁺. It is considered that severalmolecules of Pb²⁺ are coordinated to one molecule of polyethyleneglycol, preventing the association of Pb.

Example 9

The procedure of Example 6 was repeated, except that glycerin was usedinstead of polyethylene glycol as the association preventive. As aresult, a 100 nm-thick, dense, thin layer of PZT free from precipitationof a material in a stripe form was formed.

Further, the procedure of Example 8 was repeated. The results ofevaluation indicate that, although there is a slight difference, as withExample 6, the association of lead could be effectively prevented.

Example 10

The procedure of Example 6 was repeated, except that diethylene glycolwas used instead of polyethylene glycol as the association preventive.As a result, a 100 nm-thick, dense, thin layer of PZT free fromprecipitation of a material in a stripe form was formed.

Further, the procedure of Example 8 was repeated. The results ofevaluation indicate that, although there is a slight difference, as withExample 6, the association of lead could be effectively prevented.

Example 11

In this example, lead nitrate (Pb(NO₃)₂), zirconium oxynitrate(ZrO(NO₃)₂), and titanium isopropoxide (Ti(O-i-C₃H₇)₄), were used asstarting compounds.

At the outset, 4.03 g of titanium isopropoxide was dissolved in andmixed with 23.3 g of a 2.8 N aqueous nitric acid solution while stirringin a nitrogen atmosphere to give an aqueous titanium solution.Separately, 4.27 g of zirconium oxynitrate was dissolved in and mixedwith 18 g of pure water while stirring to give an aqueous zirconiumsolution. After the preparation of the aqueous titanium solution and theaqueous zirconium solution, these aqueous solutions were mixed togetherwhile stirring to prepare a homogeneous solution. Separately, 10 g oflead nitrate was dissolved in and mixed with 32 g of pure water whilestirring. The resultant aqueous lead solution was then mixed with themixed aqueous solution containing titanium and zirconium while stirring.Finally, 10% by weight, based on the resultant mixture, of polyethyleneglycol (weight average molecular weight=200) was added to the mixture.Thus, a homogeneous aqueous PZT precursor solution was prepared.

In the aqueous PZT precursor solution prepared above, the PZTconcentration was 9% by weight, and the composition (molar ratio ofmetal elements contained) was Pb:Zr:Ti=1:0.53:0.47. Among a number ofPZTs, the PZT having this molar ratio offered the maximum valuesrespectively for the permittivity, the piezoelectric constant and thelike.

To the aqueous PZT precursor solution were added a PZT powder having thesame composition as the PZT and hydroxypropyl cellulose (as a binder)each in an amount of 10% by weight based on the aqueous PZT precursorsolution. The mixture was milled by means of a planetary ball mill for 3min to prepare a PZT precursor paste.

The PZT precursor paste thus prepared was coated on a Pt/Ti/Si substrateby using a metal mask and a doctor blade, and the coating was dried at150° C. for 10 min, degreased at 500° C. for 60 min, and fired at 800°C. for 60 min. As a result, a 3 μm-thick, dense, thin layer of PZT wasformed. The thin layer of PZT was examined and found to be free fromcracking.

Thin layers of PZT having varied thicknesses (2 to 4 μm) were formed inthe same manner as described above, except that the amount ofpolyethylene glycol (used in an amount of 10% by weight in the abovecase) and the amount of the PZT powder (used in an amount of 10% byweight in the above case) were varied. As a result, as with the abovecase, crack-free, dense, thin layers of PZT could be formed.

FIG. 15 is a microphotograph (magnification: 20×) showing the surfaceappearance of the thin layer of PZT (thickness 3 μm) thus formed. Fromthis microphotograph, it is apparent that the surface of the thin layerof PZT is free from cracks, pinholes, and material in a stripe form.

Further, in order to evaluate a change in the aqueous PZT precursorsolution with the elapse of time, the aqueous PZT precursor solution wasallowed to stand in the air at room temperature for one month. As aresult, no change occurred. Since polyethylene glycol used as theassociation preventive in this example has already been found, byTG-DTA, to be a material which could be degreased at 300° C., it isconsidered that degreasing could be fully achieved in the step ofdegreasing.

Example 12

In order to evaluate the influence of an association preventive on lead(Pb) in an aqueous PZT precursor solution, the procedure of Example 11was repeated, except that the amount of polyethylene glycol used as theassociation preventive was reduced from 10% by weight to 5% by weight.As a result, unlike Example 11 (amount of association preventive used:10% by weight) wherein no association of lead was observed, in thisexample (amount of association preventive used: 5% by weight), theassociation of lead was significant. The large number of defects in astripe form appearing on the microphotograph (magnification: 20×) of thethin layer of PZT shown in FIG. 16 demonstrates the association of lead.Regarding the prevention of the association of Pb, the description inExample 8 is true of this example.

Example 13

The procedure of Example 11 was repeated, except that the PZT precursorpaste was coated using a spatula instead of the doctor blade. As aresult, as with Example 11, a crack-free, dense, thin layer of PZT(layer thickness 3 μm) was formed. The crystal structure of the thinlayer of PZT was examined by X-ray diffractometry and found to beconstituted by a single phase of PZT.

Example 14

The procedure of Example 11 was repeated, except that diethylene glycolwas used instead of polyethylene glycol as the association preventive.As a result, as with Example 11, a dense, thin layer of PZT (layerthickness 3 μm) free from precipitation of a material in a stripe formand a crack was formed. This demonstrates that, as with use ofpolyethylene glycol (Example 11), use of diethylene glycol as theassociation preventive results in the preparation of a highly stableferroelectric precursor and, in addition, coating of the precursor ontoa substrate followed by firing can provide a ferroelectric element whichis dense and possesses excellent piezoelectric properties.

Example 15

The procedure of Example 11 was repeated, except that glycerin was usedinstead of polyethylene glycol as the association preventive As aresult, as with Example 11, a crack-free, dense, thin layer of PZT(layer thickness 3 μm) free from precipitation of a material in a stripeform was formed. This demonstrates that, as with use of polyethyleneglycol (Example 11), use of glycerin as the association preventiveresults in the preparation of a highly stable ferroelectric precursorand, in addition, coating of the precursor onto a substrate followed byfiring can provide a ferroelectric element which is dense and possessesexcellent piezoelectric properties.

Example 16

This example demonstrates the formation of a composite thin layer ofPZT.

A first thin layer of PZT (substrate layer) was first prepared accordingto the following procedure. A solution of 0.00682 mol of lead nitrate inwater, a solution of 0.00273 mol of zirconium oxychloride in water, anda solution of 0.05 mol of potassium hydroxide in water were mixedtogether. A titanium substrate was immersed in the mixed solution,followed by hydrothermal treatment at 150° C. for 24 hr. As a result, acrystal nucleus of PZT was produced on the surface of the titaniumsubstrate. Separately, a solution of 0.00682 mol of lead nitrate inwater, a solution of 0.00273 mol of zirconium oxychloride in water, asolution of 0.00252 mol of titanium tetrachloride in water, and asolution of 0.05 mol of potassium hydroxide in water were mixedtogether, and the titanium substrate with a PZT crystal nucleus preparedin the above step was then immersed in the resultant mixed solution,followed by hydrothermal treatment at 120° C. for 48 hr. Thus, a firstthin layer of PZT having an average particle diameter of 5 μm and a verypoor surface smoothness was obtained.

Subsequently, on the first thin layer of PZT, formed on the titaniumsubstrate, was formed a second thin layer of PZT from an aqueous PZTprecursor solution by the sol-gel process according to the followingprocedure.

At the outset, 0.014 mol of titanium tetraisopropoxide was dissolved in30 g of a 3 N aqueous nitric acid solution to prepare an aqueoustitanium solution. Separately, 0.016 mol of zirconium oxynitrate wasdissolved in 20 g of pure water to prepare an aqueous zirconiumsolution. Further, 0.03 mol of lead nitrate was dissolved in 20 g ofpure water to prepare an aqueous lead solution. In this connection, itshould be noted that the ratio of Zr to Ti in PZT is closely related tothe ferroelectricity and this ratio is determined by the molar ratio ofthe corresponding metal salts. Therefore, the aqueous solutions ofrespective metal salts were prepared so that mixing was performed in adesired molar ratio.

The aqueous titanium solution and the aqueous zirconium solution werethoroughly mixed together at room temperature while stirring, and theaqueous lead solution was then added to and mixed with the mixture. Thepresent inventors have found that in practicing the present invention,the addition of aqueous metal salt solutions in the above order ispreferred. The resultant mixture was thoroughly stirred at roomtemperature and then heated under reflux at a temperature of 100° C. orabove to conduct hydrolysis. Metal ions contained in each aqueous metalsalt solution, when the aqueous solutions were mixed together at roomtemperature, merely formed metal hydroxides corresponding to respectivemetal ions, and the degree of bonding among the metal ions was small.Subsequent reflux and hydrolysis, however, permitted the metal ions,that is, titanium, zirconium, and lead ions, to be bonded to one anotherthrough oxygen. In this case, a Pb—O—Ti—O—Zr bond was formed to bringthe system to a sol. In the present invention, the aqueous solution inthis sol stage is an aqueous PZT precursor solution referred to in thepresent invention.

The temperature of the aqueous precursor solution was returned to roomtemperature, water and a small amount of hydroxypropyl cellulose wereadded dropwise thereto, followed by thorough mixing with stirring. Inthis case, the purpose of adding hydroxypropyl cellulose is to increasethe viscosity of the aqueous PZT precursor solution having relativelylow viscosity and to enable the formation of a coating having largethickness in the coating of the aqueous PZT precursor solution on thesubstrate. Not only has hydroxypropyl cellulose the effect of increasingthe viscosity, but also the effect of strengthening the Pb—O—Ti—O—Zrbond and the effect of reducing the distance among the ions. This canlower the firing temperature in the layer formation and in addition canimprove the crystallinity of the layer. The aqueous PZT precursorsolution after the addition of hydroxypropyl cellulose was atransparent, homogeneous solution which was neither cloudy nor containedany precipitate.

The aqueous PZT precursor solution thus prepared was spin-coated ontothe substrate with the first thin layer of PZT formed above. In thiscase, the aqueous PZT precursor solution coated on the substrate withthe first thin layer of PZT is in the uncrystallized state. Therefore,the coating was dried at 150° C. for 10 min and once fired at atemperature of 500° C. or above to decompose hydroxypropyl cellulose asthe additive.

After the completion of the firing for the decomposition ofhydroxypropyl cellulose, the substrate was fired at 650° C. for one hr,thereby forming a second PZT structure having significantly enhancedsurface smoothness. Thus, a more dense, composite thin layer of PZT wasobtained which had no boundary between the first thin layer and thesecond thin layer.

In the production of an ink jet head, the thin layer of PZT, which hasbeen formed through the above series of steps, has good adhesion to adiaphragm and an electrode to be jointed to a piezoelectric elementcomprising the thin layer of PZT, so that they can be integrally bondedwithout the aid of any adhesive. Use of the hydrothermal synthesis withthe sol-gel process can bring the firing temperature to a relativelylower value than that in the case of the conventional solid phaseprocess. Therefore, the range of selection of the diaphragm as thesubstrate can be broadened.

Example 17

The procedure of Example 16 was repeated, except that the second thinlayer of PZT was formed according to the following procedure.

0.014 mol of titanium tetrachloride was dissolved in aqueous ammonia,and the resultant titanium hydroxide was collected by filtration andwashed. The collected titanium hydroxide was dissolved in a 3 N aqueousnitric acid solution to prepare an aqueous titanium solution.Separately, 0.016 mol of zirconium oxynitrate was dissolved in 20 g ofpure water to prepare an aqueous zirconium solution. Further, 0.03 molof lead nitrate was dissolved in 20 g of pure water to prepare anaqueous lead solution.

The aqueous titanium solution and the aqueous zirconium solution werethoroughly mixed together at room temperature while stirring. Theaqueous lead solution was added to and mixed with the mixture. The mixedsolution was thoroughly stirred at room temperature and heated underreflux at a temperature of 100° C. or above to conduct hydrolysis. Thetemperature of the resultant mixed solution after the hydrolysis wasreturned to room temperature, water and a minor amount of hydroxypropylcellulose were added dropwise thereto, followed by thorough mixing withstirring. As a result, a homogeneous PZT solution was obtained.

The PZT solution thus prepared was spin-coated onto the titaniumsubstrate with the first thin layer of PZT formed above. The coating wasdried at 150° C. for 10 min and once fired at a temperature of 500° C.or above to decompose hydroxypropyl cellulose as the additive. Finally,firing was performed at 650° C. for one hr, thus completing theformation of a second thin layer of PZT.

As with the composite thin layer of PZT prepared in Example 16, thecomposite thin layer of PZT prepared in this example was satisfactory.

INDUSTRIAL APPLICABILITY

According to the present invention, aqueous solutions of respectivemetal oxides constituting a ferroelectric material are prepared andmixed together to prepare a mixed solution as a ferroelectric precursor,and additives characteristic of the present invention, such as athickener and an association preventive, are added. This constitutioncan offer many advantages unexpected from the prior art. For example,according to the present invention, the use of the aqueous metal oxidesolution as the ferroelectric precursor is advantageous in that handlingand preparation are easy and, in addition, the storage stability of thestarting compound solution is excellent, addition of an associationpreventive can enhance the stability of the precursor, a ferroelectricelement can be easily produced at low cost and can be formed as a thinlayer, and, since particles on the surface of the thin layer are fineand dense, excellent surface smoothness can be realized.

Further, according to the present invention, addition of a ferroelectricpowder, an organic binder and the like to prepare a precursor pastefollowed by the formation of a thin layer of a ferroelectric can preventthe creation of defects, such as precipitation of a material in a stripeform, on the surface of the thin layer of a ferroelectric, can provide adense, homogeneous, thin layer of a ferroelectric, ensuring excellentpiezoelectric properties. In addition, this can prevent the creation ofcracking on the surface of the thin layer.

Furthermore, according to the present invention, stacking of a secondthin layer of a ferroelectric prepared by the sol-gel process on a firstthin layer of a ferroelectric prepared by hydrothermal synthesis enablesthe thickness of the ferroelectric element to be increased whilemaintaining the features of the ferroelectric element of the presentinvention and, in addition, can offer good adhesion to the underlyingsubstrate.

In addition, application of the ferroelectric element of the presentinvention as a piezoelectric element to a piezoelectric ink jet headcan, of course, offer excellent piezoelectric properties and, further,permits an electrode to be easily formed on the piezoelectric elementwithout creating any defect.

What is claimed is:
 1. A process for producing a ferroelectric elementcomprising a layer of ferroelectric material containing at least twometals formed on a substrate, said process comprising the use of asol-gel method and including the steps of: dissolving a salt of each ofthe metals in an aqueous solvent consisting essentially of water to forman aqueous solution containing the respective hydroxide of each of themetals, with the proviso that, if titanium is used as one of the metals,the titanium salt is dissolved in an aqueous acid solution to formtitanium hydroxide: mixing the resulting aqueous solutions of respectivehydroxides of the metals together to prepare an aqueous ferroelectricprecursor solution; adding a thickener and/or an association preventiveto the aqueous ferroelectric precursor solution and coating theresultant solution onto the substrate; and drying and firing theresulting coating to crystallize the ferroelectric material and form theferroelectric element.
 2. The process according to claim 1, wherein thethickener is a water-soluble polymeric material and can beheat-decomposed in the step of crystallizing the ferroelectric material.3. The process according to claim 1, wherein the association preventivecomprises a water-soluble polyhydric alcohol.
 4. The process accordingto claim 1, wherein a powder of another ferroelectric material having acrystal structure identical or similar to the ferroelectric material isfurther added in the step of adding the thickener and/or the associationpreventive.
 5. The process according to claim 1, which further comprisesthe step of adding the ferroelectric precursor in the form of a pastederived from the aqueous ferroelectric precursor solution.
 6. Theprocess according to claim 1, which further comprises the step offorming on the substrate a substrate layer, having a crystal structureidentical or similar to the ferroelectric material, from aqueoussolutions of metals necessary for the formation of the substrate layerby hydrothermal synthesis.
 7. The process according to claim 1, whereinthe ferroelectric material is a ceramic having a perovskite structureand one of the metals is lead.
 8. The process according to claim 7,wherein the ceramic is lead zirconate titanate.